The present invention relates to a developing unit in use with an image recording apparatus based on the xerography process, such as a copying machine or a printer. More particularly, the invention relates to a developing unit which can provide a quality image of sufficiently high density.
In the xerography-basis image recording apparatus and the electrostatic image recording apparatus, there is known a process to visualize a latent electrostatic image on the image bearing member by using a two-component developer consisting of non-magnetic toner and magnetic carriers. In the process, a magnetic brush consisting of strings of magnetic carriers that are like ears is formed on the developing roll. The sleeve is rotated above the surface of the developing roll, to thereby transport the magnetic brush to a location where it faces the latent electrostatic image bearing member. The non-magnetic toner is transferred onto the latent image on the latent electrostatic image bearing member.
A developing roll as shown in FIG. 9 is generally used for the developing unit. The developing roll includes mainly a magnet roll 61 and a sleeve 60 that is rotatable with respect to the magnet roll 61. The magnet roll 61 is formed by sintering powdery ferromagnetic material, such as hard ferrite, or molding a mixture of powdery ferromagnetic material and binder into a cylindrical body. A plural number of magnetic poles are circumferentially arrayed therein, each magnetic pole being uniform in the axial direction. A support shaft, which is extended passing through the central part of the magnet roll 61, fixedly supports the magnet roll 61. Both ends of the support shaft, extended from both sides of the magnet roll 61, are supported by a housing of the developing unit. The sleeve 60 is a tubular member made of non-magnetic material, such as aluminum alloy or stainless steel. The sleeve 60, driven for rotation, is supported by a supporting member that is rotatably mounted on the support shaft.
The two-component developer is attracted to the circumferential, outer surface of the sleeve 60 through the magnetic interaction by the magnet roll 61, so that a magnetic brush is formed on the sleeve 60. With rotation of the sleeve 60, the magnetic brush is moved to the developing region where non-magnetic toner is transferred onto a latent electrostatic image bearing member 40 in an electric field present between the sleeve 60 and the latent electrostatic image bearing member 40, thereby developing the latent electrostatic image.
The magnetic brush, as shown in FIG. 10, is formed on the sleeve, along the magnetic lines of force between the adjacent S and N poles of the magnet roll 61. In the figure, the distributions of magnetic flux developed from only the outer surface area of the magnet roll 61 where it faces the latent electrostatic image bearing member 40 are illustrated. As known, the remaining outer surface develops magnetic fluxes in similar fashion. The magnetic field is the most intensive at the center of a magnetic pole that develops the magnetic flux. The magnetic flux developed from one magnetic pole is extended toward its adjacent other magnetic poles. Accordingly, the magnetic ear formed along the line prolonged perpendicularly from the center of the magnetic pole is the highest, and the magnetic ear is the lowest in the middle between the adjacent magnetic poles. For this reason, the magnetic poles are usually arrayed in the magnet roll 61 such that the developing process is carried out in a region containing the highest magnetic ear and its near ears.
In the developing process, some techniques for increasing a density of the image are known. The first technique is to turn the developing roll at high speed. The second technique is to reduce the gap between the developing roll and the latent electrostatic image bearing member. The third measure is to enlarge the developing region. Those techniques have the following problems. In the first technique based on the high speed rotation of the developing roll, toner is scattered within the machine. In the second technique based on the gap reduction, the developer tends to agglomerate under a pressure, and in an extreme case, it will damage the latent electrostatic image bearing member. The third technique, which is based on the enlargement of the developing region, is realized by using a plural number of the developing rolls, widening the magnetic poles, and the like. Where the plural number of the developing rolls are used, the machine size and the cost to manufacture as well are increased. Where the magnetic poles are widened, the machine size is not increased, but the magnetic flux density is increased. Under the increased magnetic flux density, the developer attaches in a large brush onto the sleeve, it receives a pressure from the latent electrostatic image bearing member 40, and hence tends to agglomerate.
A technique capable of preventing the agglomeration of the developer and the increase of the machine size is proposed. In the proposed technique, the magnetic poles are arranged in the developing roll so that the magnetic field has a plural number of peaks of its intensity in the developing region. The technique is realized in a manner that two magnets of the same polarity, arrayed parallel to each other, are disposed in the portion of the developing roll where it faces the latent electrostatic image bearing member. The same is also realized in a manner that a magnet is disposed in the developing region while facing the latent electrostatic image bearing member, and at least one dent is formed on the side of the magnet where it faces the latent electrostatic image bearing member (U.S. Pat. No. 4,331,100 and Published Unexamined Japanese Patent Application No. Hei. 3-291,680).
The technique in which the plural number of intensity distribution peaks of the magnetic field are formed in the developing region has the following problems. Careful and troublesome work is required for fixing the two parallel magnets at the designed locations. This leads to increase of cost to manufacture. Further, a magnetic field, which is opposite to the intended magnetic field, is developed in the gap between the two magnets. By the unwanted magnetic field, toner attaches to portions in the latent image which should not be developed. The resultant image is not clear.
In the approaches of U.S. Pat. No. 4,331,100 and Published Unexamined Japanese Patent Application No. Hei. 3-291,680, the dent is formed on the side of the magnet where it faces the latent electrostatic image bearing member. With provision of the dent, a magnetic field on the surface of the sleeve abruptly changes at the location of the dent on the magnet. In other words, a magnetic field gradient is great on the outer surface of the sleeve. In such a magnetic field, the magnetic brush formed on the sleeve is abruptly agitated, so that toner is scattered. The shape and size of the dent greatly affects an intensity distribution of the magnetic field. Because of this, to obtain a profile of the intensity distribution of the magnetic field as designed, the dent must be formed by a high precision work. To this end, a highly skilled technique is required. The result is increase of cost to manufacture.